The present invention relates generally to hydraulically-actuated fuel injectors, and more specifically to stepped top pistons and rate shaping in hydraulically-actuated fuel injectors.
It has long been known in the art that the power, efficiency and exhaust emissions of fuel injected internal combustion engines are significantly dependent upon various fuel injection parameters, including injection flow rate and variation in injection flow rate during the injection cycle. Of particular importance to our society is the reduction of undesirable engine emissions. It has thus been sought in the art to control fuel flow rate during the injection cycle by various schemes, sometimes referred to as rate-shaping, in reference to the profile of a plot of fuel flow rate versus time during an injection cycle.
Many attempts at rate shaping have been successfully implemented in hydraulically-actuated fuel injectors. Typically, the basis of operation of such injectors is as follows: a moderately high pressure actuation fluid is directed to act upon an intensifier piston with a relatively large hydraulic surface area. The piston in turn acts upon a plunger, which pressurizes the fuel. The piston has a much greater hydraulic area than the plunger; by virtue of the relative hydraulic surface areas, the fuel pressure may be magnified to many times that of the actuation fluid pressure. In other words, fuel injection pressure is proportional to the product of the actuation pressure and the intensifier piston/plunger hydraulic area ratio. While these types of injectors have been in use for many years, engineers are continuously looking for ways to improve their performance through rate shaping.
It has been found in the art that engine emissions can be significantly reduced at certain operating conditions by controlling the injection rate shape. A preferable rate shape may be generally characterized by a relatively low fuel flow rate at the beginning of the injection cycle, followed by a controlled increase to a peak flow, and with an abrupt termination of fuel flow at the end of the cycle. Such a rate shape has been found to reduce undesirable engine emissions at some operating conditions.
One method for implementing a rate-shaping scheme is discussed in U.S. Pat. No. 5,826,562 to Chen et al, which recognizes that front end rate shaping may be implemented, in one embodiment, through the use of a stepped intensifier piston. Such a piston includes two hydraulic surfaces that are separated by a cylindrical portion. The smaller diameter piston portion, referred to as the top hat portion, sits on top of the larger diameter portion of the piston. The end face of the top hat portion constitutes the top hat""s smaller hydraulic surface, and the exposed portion of the face of the larger diameter portion of the piston, annular in shape, constitutes the larger hydraulic surface. Such a stepped intensifier piston operates in a stepped bore comprising two concentric cylindrical surfaces, designed such that relatively close diametrical clearances exist between the stepped intensifier piston cylindrical surfaces and the stepped bore cylindrical surfaces. The stepped bore is further designed such that when its annular face is in contact with the exposed annular face surface of the larger diameter portion of the piston, which condition is a result of forces applied by the piston""s return spring, a slight gap exists between the smaller diameter bore""s end face and the piston""s top hat portion hydraulic surface.
In such a system, the actuation fluid is typically provided via a port in the end face of the smaller diameter portion of the bore. Accordingly, during the beginning of an injection cycle the actuation fluid acts primarily on the top hat hydraulic surface, and results in a lower force acting upon the intensifier piston against the piston return spring and the plunger chamber fuel pressure, such that a certain corresponding fuel injection pressure and flow rate is achieved. As the injection cycle proceeds, the intensifier piston is driven further from its seated position, and the top hat piston portion eventually clears the small diameter portion of the bore, exposing the annular hydraulic surface of the larger diameter piston portion to the actuation fluid pressure. This increases the effective hydraulic surface area of the intensifier piston, resulting in an increased force being applied to the intensifier piston, and an increase in the fuel pressure and flow rate. Thus, during operation, the intensifier piston initially moves at a relatively slow speed, providing a relatively low injection rate, and later during the injection cycle, i.e., after the top hat piston portion clears the small diameter portion of the bore, accelerates to a greater speed, providing a relatively higher fuel injection rate. Notwithstanding the improvements in rate shape provided by this scheme, further improvements are possible.
As previously stated, the intensifier piston begins moving from its seated position at a relatively slow rate during the initial portion of the injection cycle, and during the latter portion of the cycle, accelerates to a greater speed. Necessarily, the greater the piston speed, the greater the actuation fluid flow rate required to maintain or to continue to accelerate that piston speed. Once the top hat piston portion begins to clear the small diameter portion of the bore, an increase in overall piston hydraulic area is exposed to actuation fluid pressure. As the piston continues to move downward in its stroke, the clearance between the top hat piston portion and the small diameter portion of the bore increases, exposing the piston""s larger annular hydraulic surface to more actuation fluid. This causes more acceleration of the piston due to the actuation pressure acting on a larger area and thus providing a larger net force to the intensifier piston. The greater piston acceleration results in a higher fuel pressure, hence, the more the piston can be accelerated, the higher the peak fuel pressure. At this stage in the injection cycle, the required actuation fluid flow rate must increase sharply, relative to the flow rate during the initial portion of the injection cycle, to compensate for the accelerated motion of the piston. In order for the piston to maintain its higher speed or to accelerate to a greater speed, actuation fluid must be supplied at the same pressure, but with a greater flow rate. In order to improve injector performance, a means to provide an increase in available actuation fluid flow, at a designated point in the intensifier piston stroke, approximately in the vicinity of where the top hat piston portion clears the small diameter portion of the bore, is desirable. Thus, although rate shaping with a stepped piston has proven a viable concept, there exists room for improvement.
The present invention is directed towards overcoming these and other problems, and to improving the rate shaping performance and maximum fuel pressure in hydraulically-actuated fuel injectors.
A hydraulically actuated fuel injector includes an injector body defining an actuation fluid cavity that is fluidly connected to a piston bore via a plurality of actuation fluid passages. An intensifier piston having a side surface and a top, including a first hydraulic surface that is separated from a second hydraulic surface, is positioned in the piston bore, and is moveable a stroke distance between a retracted position and an advanced position. The first hydraulic surface of the intensifier piston is exposed to fluid pressure in a first cavity over a beginning portion of the intensifier piston""s stroke distance via a relatively unrestricted first passage of the plurality of actuation fluid passages. The second hydraulic surface of the intensifier piston is exposed to fluid pressure in a second cavity over a beginning portion of the intensifier piston""s stroke distance, via a second passage of the plurality of actuation fluid passages, having relatively restricted flow area. The injector body includes a third passage of the plurality of actuation fluid passages, having relatively unrestricted flow area, which is blocked by the side surface of the intensifier piston over a portion of its stroke distance.
In another aspect of the invention, a directly controlled fuel injector includes an injector body defining a nozzle outlet, a needle control passage, and an actuation fluid cavity that is fluidly connected to a piston bore via a plurality of actuation fluid passages. An intensifier piston, having a side surface, and a top surface which includes at least one hydraulic surface, is positioned in the piston bore, and is moveable a stroke distance between a retracted position and an advanced position. The injector body includes a passage of the plurality of actuation fluid passages, having relatively unrestricted flow area, which is blocked by the side surface of the intensifier piston over a portion of its stroke distance. The directly controlled fuel injector also includes a needle valve member that is positioned in the injector body adjacent to the nozzle outlet. The needle valve member includes a closing hydraulic surface that is exposed to the fluid pressure in the needle control passage.
Yet another aspect of the invention is a method of front end rate shaping in a hydraulically actuated fuel injector. Realization of this method includes the step of driving an intensifier piston of a hydraulically actuated fuel injector with a small hydraulic force over a beginning portion of its stroke. This is accomplished in part by covering one of the plurality of actuation fluid passages with the intensifier piston during the beginning portion of its stroke. This method also includes the step of opening a nozzle outlet of the fuel injector, accomplished in part by relieving hydraulic pressure on a closing hydraulic surface of a needle valve member. This method additionally includes the step of driving the intensifier piston with a large hydraulic force for an other portion of it stroke, accomplished in part by moving the intensifier piston to a position that uncovers the one actuation fluid passage. This method further includes the step of closing the nozzle outlet, accomplished in part by resuming hydraulic pressure on the needle valve member""s closing hydraulic surface.